1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiple Access (OFDMA) communication system and, in particular, to an Inter-Symbol Interference (ISI) cancellation technique for an OFDMA communication system.
2. Description of the Related Art
Typically, an OFDMA communication system uses a hard handover. Recently, various soft handover techniques have been proposed for the OFDMA system due to the advantages of the soft handover, such as a diversity gain and an improved Carrier-to-Interference-and-Noise Ratio (CINR) at cell edges in a cellular network of which the frequency reuse factor is 1.
In the cellular system of a frequency reuse factor of 1, the soft handover is performed by assigning symbols and subcarriers in various schemes.
In a first scheme, base stations involved in a handover transmit the same signals through an identical timeslot and identical subcarrier. In this case, a mobile station receives the signals from the two base stations without distinction. The two base stations also transmit the same resource allocation information or mobile application part (MAP), and no reception problem occurs if the mobile terminal receives the resource allocation information from at least one of the two base stations. This provides an effect for reducing frequency shadow areas and increasing reception CINR.
In a second scheme, the two base stations transmit the same signals through the same timeslot and different subcarriers. In this case, the mobile terminal needs to receive the resource allocation information from both base stations. This second scheme has an advantage of improved frequency diversity gain.
In a third scheme, the two base stations transmit the same signals through different timeslots and different subcarriers. In this scheme the mobile terminal also needs to receive the resource allocation information from both base stations for receiving the signals.
In order to recover the received signal through the same or different timeslots and subcarriers, the symbol timing of each base station needs to be synchronized in a range of a Cyclic Prefix (CP) of the OFDMA symbol. If any symbol is delayed longer than the length of the CP, adjacent symbols interfere with each other, i.e. ISI occurs, resulting in reduction of the reception CINR.
Typically, a frame synchronization of the transmit antennas of each base station is supported by a Global Positioning System (GPS). However, different round trip delays (RTD) and multipath effects between the mobile station and the base stations cause delay spread, whereby the symbol synchronization offset may become greater than the CP.